Synchronous alternating current electrical machines

ABSTRACT

A synchronous AC generator has a DC excited superconducting field winding on the rotor and a nonsuperconducting armature winding on the stator. The armature winding is composed of active conductors and of reactance conductors connected in series with the active conductors and disposed in a magnetic shield surrounding the active conductors whereby the magnetic shield required to prevent large fields outside the machine due to the superconducting field winding also serves to provide the reactance necessary in the reactance conductors to limit short circuit currents.

United States Patent Hughes 5] Feb. 22, 1972 [54] SYNCHRONOUSALTERNATING 3,242,4l8 3/1966 Mela ..3l/40 CURRENT ELECTRICAL MACHINES3,454,857 7/1969 Farrand ..3l0/46 3,470,408 9/1969 Lewis.........3l0/l55 Inventor: Austin g Fossway, Newcastle up 3,15 1,261 9/1964Lee ....3 10/165 y e, E g 2,644,905 7/1953 Brueder .310/154 Assignee:gitemalionlal m Devehnrment FOREIGN PATENTS OR APPLICATIONS v t 332:?ewcas e yne 1,023,930 3/1966 Great Britain 1.3 /169 Filedl g- 9 1970Primary Examiner-J. D. Miller Assistant Examiner-41. Skudy [211 App]'61398 Attorney-Kemon,Palmer&Estabrook 301 Foreign Application PriorityData 1 [571 ABSTRACT Aug. 1969 Great Britain "41 630/69 A synchrdmus ACgenera has a DC excited supemnduc" ing field windingon the rotor and anonsuperconducting armature winding on the stator. The armature windingis comil ..3l0/165, 310/li';)623k1(1)l9 posed of active conductors andof madame conductors com nected in series with the active conductors anddisposed in a [58] FIG"! of Search ..310/l65, 40, 46, 44, 160, 161,magnetic shield Surrounding the active conductors h b 310/165 6 153, 21the magnetic shield required to prevent large fields outside the 207,254i 258, 52; 322/28 machine due to the superconducting field windingalso serves to provide the reactance necessary in the reactance conduc-[56] References Cited tors to limit short circuit currents.

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Q o s 3 SYNCHRONOUS ALTERNATING CURRENT ELECTRICAL MACHINES The presentinvention relates to a synchronous alternating current machine having aDC-excited rotor field winding of superconducting material and a statorarmature winding of nonsuperconducting material.

The use of a superconducting field winding eliminates the need for astator core of magnetic material and the stator conductors may besupported in air or in a casing of nonmagnetic material such as epoxyresin. Because of the high magnetic field intensities produced, however,it is necessary to have a magnetic shield surrounding the rotor but thisis usually spaced well away from the rotor so as to lie outside theareas of high flux density.

Large machines of this kind can provide problems under short-circuitconditions as the currents flowing under such conditions may besufficient to damage the windings severely.

Short-circuit currents can be reduced by using magnetic material such asiron in the high flux regions, but the need to use such materialdetracts from one of the main advantages of a superconducting winding inthat a magnetic core can be dispensed with.

In accordance with the present invention the stator winding has anactive portion formed by conductors supported in a nonmagneticenvironment adjacent the rotor, a further portion formed by conductorslocated in a magnetic shield surrounding the active portion and therotor, and end connections connecting conductors of the active andfurther portions in series.

The further portion of the winding associated with the magnetic materialof the magnetic shield provides sufficient reactance to limit shortcircuit currents without the necessity of providing any additionalmagnetic material around the active portion of the winding. Theconductors of the active portion of the winding can be carried by acasing of nonmagnetic material spaced from the magnetic shield.

The conductors of the active and further portions of the windingpreferably extend parallel to the axis of the rotor and are joined byend connections extending generally radially with respect to the rotoraxis. In a preferred construction the conductors of the active portionof the winding form a doublelayer winding, the end connections are atone end of the machine, and the further winding portion has a number ofpoles differing from that of the active portion.

The conductors of the further portion can be located in slots in alaminated magnetic shield or in an annular gap between two cylindersoflaminations forming the shield.

The invention will now be described in more detail with the aid ofexamples illustrated in the accompanying drawings, in which:

FIG. I is a longitudinal section of a synchronous AC machine inaccordance with the invention,

FIG. 2 is a partial section on the line II-II of FIG. 1,

FIG. 3 is a section similar to that of FIG. 2 showing altemativearrangements of the stator winding in the magnetic shield, and

FIG. 4 is a schematic winding diagram of the stator winding ofthemachine of FIG. 1.

The machine shown in FIG. 1 is a synchronous alternating currentgenerator having a hollow steel rotor within which is supported a directcurrent field winding 11 ofsuperconducting material. The rotor 10 hasstub shafts l2 and 13 at each end which are supported in bearings 14 and15, respectively, carried by brackets 16 and 17, respectively. The stubshaft 13 has a coupling flange 18 for coupling it to a prime mover suchas a turbine for driving the rotor. The stub shaft 12 carries slip rings19 which are engaged by brushes 20 and are connected by leads 21 to thefield winding 11 to supply current to the winding. A refrigerating unit22 is also mounted on the stub shaft 12 to supply coolant at very lowtemperatures of a few degrees absolute to the winding 11 to maintain itin a superconducting state. The rotor 10 has a radiation shield 23surrounding its cylindrical surface to reduce the inflow of heat to thewinding. The radiation shield 23 is enclosed in a vacuum space within anouter casing 24.

The stator of the machine has end members 25 and 26 and a mild steelouter casing 27. Torque rings 28 of nonmagnetic alloy are mountedbetween the casing 27 and the end members 25 and 26 and an annular mainbody 29 also of nonmagnetic alloy is bolted to the torque rings 28.Active conductors 30 of the stator windings are carried on the innerface of the main body 29. A magnetic shield 31 of laminated constructionand of hollow cylindrical form is mounted around the main body 29between the torque rings 28. Reactance conductors 32 pass through theshield 31 and as shown in FIG. 2 lie in an annular gap 33 between twocoaxial cylinders forming the shield. The reactance conductors 32 areconnected in series with the active conductors 30 by means of endconnections 34 which extend generally radially in a plane transverse tothe axis of the rotor. The junctions between the active conductors 30and the end connections 34 are enclosed in main water boxes 35 and thejunction between the end connections 34 and the reactance conductors 32in auxiliary water boxes 36. The water boxes 35 and 36, which areannular in form, serve to cool the conductors, which operate at or aboveroom temperature, that is they are not superconducting.

The magnetic shield 31 shown in FIG. 2 is composed of coaxial cylinderseach made up oflaminations of silicon steel. The reactance conductors 32which are located in the annular gap between the two cylinders can beembedded in epoxy resin or the like. It is also possible to support theactive conductors 30 by means of members of reinforced epoxy resininstead of the body 29 of nonmagnetic alloy. These active conductors arelocated in a nonmagnetic environment and are spaced from the magneticshield 31.

FIG. 3 shows alternative arrangements of the reactance conductors inslots in the magnetic shield 31. Reactance conductors 32A can be locatedin slots in the outer surface of the magnetic shield or reactanceconductors 328 can be located in slots in the inner surface. A furtheralternative is to locate the conductors in drilled holes in the magneticshield.

FIG. 4 shows the winding diagram for a three-phase winding in which theactive portion is a two-pole double layer winding and the reactanceportion is an eight-pole winding with only four pole-phase groups. Thefirst phase winding is shown by a thick line extending between terminals40A and 403, the second phase winding by a thin line extending betweenterminals 41A and 41B, and the third phase winding by a broken lineextending between terminals 42A and 423. At 43 are shown the activeconductors lying in pairs in slots to form the double winding. In eachpair one conductor, for example the conductor 44, is shown in the mannerspecified above and lies over another conductor, for example theconductor 45, which is shown broken (or in the case of the third-phasebroken line as a chain dotted line). The end connections are shown at 46and in this case lie at one end of the machine only. The reactanceconductors are shown at 47 and it will be seen that they are connectedin series with the active conductors by way of the end connections.

It will be noted that the active and reactance portions of the windinghave different numbers of poles. The reactance portion is an eight-poleportion but half the pole-phase groups have been omitted, leaving onlyfour. In omitting pole-phase groups the reactance portion must, ofcourse, remain balanced between phases.

Iclaim:

1. A synchronous alternating current electrical machine comprising arotor and a stator, a field winding of superconducting material on saidrotor, means for supplying direct current to said field winding, meansfor maintaining said field winding at superconducting temperatures, anarmature winding of nonsuperconducting material on said stator, and amagnetic shield on said stator, wherein said stator winding comprisesactive conductors disposed in a nonmagnetic environment adjacent therotor, reactance conductorsdisposed in said magnetic shield, and endconnections connecting the reactance conductors and the activeconductors in series, said stator comprising nonmagnetic meanssupporting said active conductors and said magnetic shield surroundingsaid active conductors and said rotor.

2. An electrical machine as claimed in claim 1 in which the active andreactance conductors extend generally parallel to the axis of the rotorand the end connections extend in a plane transverse to said axis.

3. An electrical machine as claimed in claim 1 in which the activeconductors form a double layer winding.

4. An electrical machine as claimed in claim 1 in which the portion ofthe armature winding formed by the reactance conductors has a number ofpoles different from that of the portion formed by the activeconductors.

5. An electrical machine as claimed in claim 1 in which the magneticshield comprises two coaxial cylinders of magnetic laminations having agap therebetween and the reactance conductors are disposed in the saidgap.

6.An electrical machine as claimed in claim 1 in which the magneticshield has slots in which the reactance conductors are mounted.

7. An electrical machine as claimed in claim 1 wherein the annaturewinding is a three-phase winding, the active conductors extend parallelto the axis of rotation of the rotor and are arranged in pairs to form adouble layer winding, the reactance conductors extend parallel to theaxis of rotation of the rotor and the end connections are disposed atone end of the armature winding and so connect the active and reactanceconductors in series that the portions of the winding formed by theactive and reactance conductors have different num bers of poles.

1. A synchronous alternating current electrical machine comprising arotor and a stator, a field winding of superconducting material on saidrotor, means for supplying direct current to said field winding, meansfor maintaining said field winding at superconducting temperatures, anarmature winding of nonsuperconducting material on said stator, and amagnetic shield on said stator, wherein said stator winding comprisesactive conductors disposed in a nonmagnetic environment adjacent therotor, reactance conductors disposed in said magnetic shield, and endconnections connecting the reactance conductors and the activeconductors in series, said stator comprising nonmagnetic meanssupporting said active conductors and said magnetic shield surroundingsaid active conductors and said rotor.
 2. An electrical machine asclaimed in claim 1 in which the active and reactance conductors extendgenerally parallel to the axis of the rotor and the end connectionsextend in a plane transverse to said axis.
 3. An electrical machine asclaimed in claim 1 in which the active conductors form a double layerwinding.
 4. An electrical machine as claimed in claim 1 in which theportion of the armature winding formed by the reactance conductors has anumber of poles different from that of the portion formed by the activeconductors.
 5. An electrical machine as claimed in claim 1 in which themagnetic shield comprises two coaxial cylinders of magnetic laminationshaving a gap therebetween and the reactance conductors are disposed inthe said gap.
 6. An electrical machine as claimed in claim 1 in whichthe magnetic shield has slots in which the reactance conductors aremounted.
 7. An electrical machine as claimed in claim 1 wherein thearmature winding is a three-phase winding, the active conductors extendparallel to the axis of rotation of the rotor and are arranged in pairsto form a double layer winding, the reactance conductors extend parallelto the axis of rotation of the rotor and the end connections aredisposed at one end of the armature winding and so connect the activeand reactance conductors in series that the portions of the windingformed by the active and reactance conductors have different numbers ofpoles.